Lecture 14a: Introduction to Secondary Systems Material prepared by GARD Analytics, Inc. and University of Illinois at Urbana-Champaign under contract.

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Presentation on theme: "Lecture 14a: Introduction to Secondary Systems Material prepared by GARD Analytics, Inc. and University of Illinois at Urbana-Champaign under contract."— Presentation transcript:

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Lecture 14a: Introduction to Secondary Systems Material prepared by GARD Analytics, Inc. and University of Illinois at Urbana-Champaign under contract to the National Renewable Energy Laboratory. All material Copyright U.S.D.O.E. - All rights reserved

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2 Importance of this Lecture to the Simulation of Buildings  Simulation is important, but understanding what one is simulating is equally critical  Lack of understanding (treating an HVAC system like a black box) can lead to errors in input

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3 Purpose of this Lecture  Gain an understanding of: Different types of space conditioning systems General characteristics about how they operate and what some of their advantages/disadvantages might be Prepare for upcoming lectures on EnergyPlus system input

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4 All-Air Systems: Overview  Provides all sensible and latent cooling, preheating, and humidification required by the zone(s)  Additional cooling or humidification at the zone rare (industrial systems)  Heating is either provided by the main air stream by the central system or locally at a specific zone  Classified into single- and dual-duct categories as well as constant and variable volume categories  Conditioning depends on air mass flow rate and temperature (enthalpy) difference between supply and room air

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8 Advantages vs. Other System Types  Flexible: high degree of flexibility on how to meet loads, distribute air, etc.  Low noise: most equipment kept away from occupied spaces  Control: probably the best control of both temperature and humidity can be achieved with air systems (precise control situations)  Most easily understood and popular  Indoor air quality: IAQ is part of the system (not an “afterthought”)

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9 Disadvantages vs. Other System Types  Space: require additional space for air distribution ductwork (vertically and horizontally) adds to building size in all directions  Concerns about access to terminal devices  Requires air balancing which can be difficult on large systems  Perimeter heating not always available during construction

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10 Single-Zone Constant Volume  Also known as single zone draw through system  Simplest all-air system, meets all conditioning needs of space  System can be in zone or at a remote location, with or without distribution ductwork  Little or no ductwork means low pressure drop and lower fan energy  Systems can be turned off without affecting adjacent systems

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11 Multiple-Zone Reheat (Constant Volume)  Also known as terminal reheat in some circles  Single air stream at a fixed temperature delivered to various spaces  Local supply temperature is varied by the use of a terminal reheat coil  Main duct temperature typically cooled to cold deck temperature (all year)  large amount of energy consumption  When main duct air temperature varies, could have humidity problems

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12 Constant Volume Bypass  Bypass replaces reheat coil  Total flow of system remains constant but flow of air to space is varied  Excess flow is bypassed around zone and directly into return duct  Saves the energy that would be used by the reheat coil  Requires a return fan to avoid air short circuiting back into the room from the bypass; still have contact supply fan energy  Usually only used in smaller applications where humidity control is not as important

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13 Variable Air Volume Systems  Objective: reduce flow rate when loads are not as high to save fan energy (fan energy proportional to flow rate cubed)  Especially effective for perimeter zones that may receive solar heating  Temperature is maintain same for all zones, flow rate varied to each  Flow rate bounded at lower end by a minimum air fraction  Concern 1: indoor air quality—outside air may limit lower bound of VAV flow rate  Concern 2: humidity—it may also limit lower bound of VAV flow rate  Terminal devices used to further reduce cooling or to provide heating  From maximum cooling point, VAV first throttles back flow and then adds reheat (or uses terminal device)

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14 Dual-Duct Systems: Overview  Two air streams are conditioned at a central location: one warm, one cold  Air distributed to spaces through: Two ducts (one warm, one cold) and mixed locally, disadvantage of two ducts (both must be sized to handle their maximum flow rate though this may be different for heating and cooling, cost, space) A single duct per zone after mixed centrally, disadvantage(?) of multiple ducts  Tends to require more energy than a VAV system  Dual-duct can serve one or more thermal zones  May have more than one fan (dual fan—one for each duct)

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15 Dual-Duct Constant Volume  Total air flow rate remains fixed, but volume through each duct varies with heating/cooling load  Single fan with reheat Similar to the terminal reheat system Reheat applied at a central location rather than at each individual zone Air is not cooled and then reheated as in terminal reheat Uses less energy than terminal reheat because some air is heated and other air is cooled Constant volume system—air flow is constant and thus fan energy is always same (high)

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16 Dual-Duct Constant Volume (cont’d)  Single fan, no reheat Similar to a single-duct system Simply has a cooling coil bypass and is very similar to a single duct system Does not expend energy to reheat air, simply uses a mixture of return and outdoor air Less air is sent through cooling coil, may result in less dehumidification and thus moisture problems during parts of the year (spring/fall)

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17 Dual-Duct Variable Air Volume (DDVAV)  Blends warm and cold air in variable volume combinations  Flow reduced below maximum load and flows mixed at minimum flow rate (which might be limited by outside air or humidity concerns)  Can be combined with single duct VAV systems for zones that are cooling only (interior spaces)  Dual-duct terminal units can also serve as separate VAV systems (one warm, one cold) which can reduce fan energy and heating and cooling energy

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18 Single Fan DDVAV  Fan sized for the “coincident peak” of the hot and cold deck volumes  Control via two static pressure controllers (one in each deck)  Cold deck typically kept at same temperature though it may be varied  Hot deck sometimes adjusted up when temperature outside is low or when humidity is high (to force more air through the cold deck)  Some systems may use a precooling coil for the mixed or outside air

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20 Dual Conduit  Really a “dual system” configuration  Primary air (constant flow) system: Used to meet exterior transmission (perimeter) loads Run only during peak periods Runs without outside air (can be fairly local) Must be sized to account for action of secondary system at minimum air fraction  Secondary air (variable volume) system: Runs year-round Serves both interior and perimeter spaces Used to meet the loads from people, lights, equipment, and solar heating Used to bring in outside (fresh) air Variation on this system mixes the two air streams using a dual duct terminal box—in this case the primary system is heating only and the secondary system must meet the entire cooling load

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21 Multizone Systems: Overview  Similar to a dual-duct system (though only for more than one zone)  Multizone systems share same advantages and disadvantages as dual- duct systems  Multizone systems can be obtained as “packaged” units (lower first costs) of up to approximately 12 zones

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22 Three-Deck Multizone  Has an additional duct for “unconditioned” air  This duct is in addition to the warm and cold air ducts (hence the name)  Only two of the ducts unconditioned and either warm or cold used at any given time (seasonal switchover)  Generally not used due to the extra initial expenses  Can be mimicked by a standard dual-duct by seasonally scheduling coils

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23 Summary  Single-duct systems: Main heating and cooling coils in series arrangement Ducts supply air to all terminals at a common temperature Capacity varied by varying temperature or flow rate  Dual-duct systems Main heating and cooling coils in parallel May use separate warm and cold air duct distribution systems, blending air at the terminal device May blend air near the main unit; separate duct for each zone Most vary supply temperature

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Lecture 14b: Introduction to Secondary Systems Material prepared by GARD Analytics, Inc. and University of Illinois at Urbana-Champaign under contract to the National Renewable Energy Laboratory. All material Copyright U.S.D.O.E. - All rights reserved

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25 Special Systems: Dual Air Stream System  Two interconnected all air systems (diagram)  Primary air system used for outside air  Secondary air system used to condition space  Used when there are high space gains or high air flow rates required  Both systems might have heating and cooling coils

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26 Special Systems: Under-floor Air Distribution (UFAD)  Also called “up air system”  Air distributed from floor rather than overhead  Air enters space through floor registers or through furniture  Individual control and high degree of flexibility  Floor surface is also “conditioned” (pseudo- radiant system)

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27 Special Systems: Evaporative Cooling  Water vapor added to air stream to reduce the dry bulb temperature  Raises the humidity level so this tends to only have application in drier climates  Cooling can either be direct (of supply or outside air stream) or indirect (return air with heat exchanger)  Can offset cooling needs, requires water as a resource

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28 Special Systems: Low- Temperature Systems  Concept: lower the supply air temperature for cooling so that we can reduce the flow rate (less flow means less fan energy)  Typically only used in conjunction with ice storage systems  Ice can be used to produce lower supply air temperatures  Cannot produce colder air directly using chiller because lower evaporator temperatures will make chiller less efficient  Terminal units may be required to maintain a sufficient amount of air movement within the space

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29 Terminal Units: Overview  Terminal unit is the last (hence terminal) device on the air distribution system (between the duct and the conditioned space)  Two types: Supply outlet—register or diffuser: goal is to supply air to the space without causing drafts or excessive noise Terminal unit: controls the quantity and temperature of the supply air (by further conditioning the air)

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31 Variable Air Volume Terminal Units  Terminal unit reduces the flow rate in an attempt to provide only enough cooling to match the actual cooling load  Care must be taken to avoid too much flow in areas close to the air handler

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32 Throttling Units  Throttling Unit Without Reheat Air flow rate can be throttled down to meet the cooling load Sometimes flow can be completely shutoff (air quality concerns) Concerns about noise from throttling (typically have some sort of sound attenuation)  Throttling Unit With Reheat Air flow rate can be throttled down to some minimum Below the minimum, reheat energy would be required Can also be used in cases where there are actual heating loads Flow rate reduced first, then reheat added Some systems will ramp up flow rate again once reheat turned on to avoid excessive reheat energy Reheat is sometimes replace with a baseboard unit

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34 Fan-Powered Terminal Unit  Also referred to as “fan-powered VAV box”  Basic idea is to reuse heat from space, lights, etc. to provide the “reheating” (similar to induction except we are now actively moving air with a fan)  Better circulation due to increased air movement (over throttling unit)  Parallel arrangement—local fan is outside primary air supply stream  Series arrangement—local fan is inside primary air stream and runs when space is occupied  Various heater options available to provide perimeter heating (coil, baseboard, radiant system)  During unoccupied hours, main system can be shut off and local fans can be run to meet loads as needed  Increase in fan energy over throttling units possibly offset by reuse of heat from space  Noise a potential problem since fan is so close to occupied space

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36 In-Room Terminal Systems: Main Characteristics  “Primary” or ventilation air provided either centrally or locally Central air: handles indoor air quality requirements and latent loads (humidity) Centralized maintenance and lack of moisture condensation in the terminal units Requires ductwork but typically smaller than all-air systems Air can be supplied through the terminal unit or separately from it Moisture addition/removal can be accomplished centrally  Local air: handles all conditioning requirements locally through building apertures Eliminates requirements for ducts since outside air is introduced through terminal unit Increase in maintenance costs due to outside air handling “distribution” Generally lower first costs

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37 In-Room Terminal Systems: Main Characteristics  Water supplied to local coils (“secondary water”) used to provide additional conditioning beyond that which is done by the central air Usually sensible only—latent cooling at the terminal unit would significantly decrease the life of the unit and could lead to odors/bacterial growth Any condensate is typically left in space to get reabsorbed by room air at some later time (drain usually recommended) Can be either heating or cooling  Applications Primarily exterior spaces with mainly sensible loads and no strict humidity control requirements Spaces where individual control is important or preferable Common installations: hospitals, hotels, schools, apartment buildings, office buildings, research laboratories

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40 Induction Units: Advantage  Individual temperature control on separate thermostats at fairly low cost (biggest advantage)  Ducts and air handling units can be reduced in size since much of conditioning can be done locally  Moisture addition/removal, filtering, and outside air can be done centrally  Space heating during unoccupied hours does not require fan operation  Components last relatively long (15-25 years) with limited maintenance (cleaning filters and nozzles)

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41 Induction Units: Disadvantages  Usually limited to perimeter spaces, requiring another system for interior spaces  Primary air flow is constant and flow to units cannot be shutoff individually (local coils can be shutoff)  Secondary air flow dependent on condition of lint screen and nozzle  Potentially lower chilled water temperature needed due to reduce air flow to zones and desire to avoid local cooling coil condensation  Not appropriate for spaces with high ventilation requirements since primary flow is reduced  Local moisture sources (open windows, etc.) can cause unexpected condensation  Higher initial costs and higher operating costs (due to high pressure requirements) than most air systems

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42 Fan-Coil Units: Characteristics  Components: coils, fan/fan motor, filter, insulated condensate pan/drain, controls/valves, return and supply air openings  Heating and cooling (though maybe only one at a time)  Forced convection from coil to air using local fan  Outside air locally or (usually) separate from the unit via a central source (example: high-rise hotels) Local outside air eliminates balancing problem if windows are opened Local outside air not allowed in commercial buildings because wind pressure changes outside reduce control over outside air Local outside air systems might require coil freeze protection  Water (hot and/or cold) supplied from central source  Electric heater might be needed for a two-pipe system for shoulder seasons

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43 Fan-Coil Units: Types/Locations  Vertical units Floor mounted, can also be low profile style Usually installed at the perimeter under window sill Low profile units can present maintenance problems  Horizontal units Ceiling mounted, saving floor space May use ductwork to distribute air (greater pressure required at fan)\ Can also combine air with central (outside) air Maintenance and condensate handling more difficult though initial costs lower  Chase-enclosed units Unit is typically floor to ceiling configuration More likely to see these in hotels and residential buildings Back-to-back placement with sound treatment

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44 Fan-Coil Units: Selection and Capacity/Control Issues  Common technique is to select a unit that can meet the cooling needs of a space at the medium speed setting of a three-speed unit (safety factor and less noise during most operation conditions)  Sizes can be reduced if outside air handled separately and introduced at a temperature close to room air conditions (~21  C)/reasonable humidity  Automated control on water flow rate, manual or auto control on air flow rate  On-off control not recommended (noise/circulation issues)

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45 Fan-Coil Units: Water Distribution Schemes  General considerations Pipes refer to fluid pipes entering and leaving the unit More pipes increases initial costs Less pipes requires “changeover” strategies and may result in lack of heating or cooling when needed All can be used with central ventilation

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46 Fan-Coil Units: Water Distribution Schemes  Two-Pipe One inlet pipe, one outlet pipe (single coil) Heating and cooling happen seasonally with changeover from one to the other (problems during intermediate seasons) Changeover typically determined by outdoor air temperature, all zones changeover at same time Can result in frequent changeovers Two-pipe changeover with partial or full electric strip heat can help avoid frequent changeovers by meeting smaller heating loads Changeovers not necessary if building is dominated by either heating or cooling

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47 Fan-Coil Units: Water Distribution Schemes  Three-Pipe Two inlet pipes, common return pipe (two coils) Generally not recommended since mixing the hot and cold water return is a waste of energy  Four-Pipe Two inlet pipes, two outlet pipes (two coils) Highest initial cost but best performance of fan coil units May include a deadband between heating and cooling to avoid simultaneous heating/cooling

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48 Fan-Coil Units: Advantages  Individual control of spaces/central water production  Pipes require less space than ducts (easier for retrofits?)  Potential lack of central AHU may also save space

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50 Packaged Terminal Systems  Similar in concept to fan-coil units except that cooling provided locally by “window AC” type unit  Heating can either be through central water/steam source or local (electric coil/heat pump)  Advantages/disadvantages similar to other air/water systems with additional concerns: Units have relatively short (appliance) life Concerns about outside air and water leaks into building through unit Noise from compressor can be excessive and variable

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51 Summary  Induction Units: Centrally conditioned “primary” air Bernoulli effect to draws secondary air through secondary coil  Fan-Coil Units: Forced convection from coil to air using local fan Outside air locally or separate from the unit via a central source Water (hot and/or cold) supplied from central source  Many other system types possible: packaged, low temperature, evaporative, etc.